1. Field of the Invention
This invention relates to methods and means of ablating the cladding of an optical fiber by using high energy electromagnetic beams such as lasers. Fiber optic couplers and fiber optic polarizers are formed with fibers subjected to this process.
2. Prior Art
Several techniques have been used to remove or reduce the dimensions of the cladding material covering optical fibers as one step in the production of fiber optic couplers. The optical fiber are typically about one hundred microns in diameter with a five or six micron diameter core.
The most common techniques are acid etching, grinding (or lapping) and fusing. In the acid etch technique, two or more optical fibers are twisted together. The fibers are placed in an acid bath until the claddings are etched away thus placing the core regions in close proximity. The twisted fibers are then immersed in index matching liquids so that light from one fiber can more easily enter surrounding cores.
In the grinding method, a fiber is fixed in a groove cut in a holding block. The fiber is arched so that the portion of the cladding to be removed is raised above the surface of the holding block. Typically abrasives or pastes are then employed to grind away the raised cladding and expose the core.
Two fibers prepared by grinding are placed together with the cores in close proximity to form a coupler. Index matching fluid or adhesive is sometimes used as a thin layer between cores.
In the fusing method, two fibers are pressed together and a portion of the combination is heated to melt the cladding material. The fibers are stretched when hot until the core regions are in close proximity. U.S. Pat. No. 4,054,366 issued to Barnoski, et al discloses fusing where a CO.sub.2 laser is used to melt the cladding.
All of the above methods have drawbacks. All are labor intensive and traditionaly low yield. In addition to requiring the handling of corrosives, the acid etch technique produces an end product which cannot be readily integrated in fiber optic systems.
Grinding can produce very flat surfaces but at very high cost. Further, couplers produced with fibers made by the grinding method have thermal expansion problems. A coolant is flowed over the fibers as grinding occurs. The coolant causes the growth of micro-cracks in the fiber and if the fiber is removed from the holding block, the fiber could break. Therefore the fiber is left in the holding block and the coupler formed with the holding blocks attached. The blocks are typically metal or some other material with a coefficient of thermal expansion which is different from that of the optical fiber. In operation, the different coefficients of thermal expansion can affect the relative position of the coupled fibers and thus the amount of light coupled.
Further, if a layer of refractive index matching fluid is used between cores, the fluid will usually have a different thermal coefficient of refractive index than the ground fibers. This in turn provides another source of coupling instability due to mismatched thermal coefficients.
With fused fibers, the stretching is extensive and reduces the core diameter as well as the outside diameter of the fiber. This causes a broadening of any lightwave which propogates along the core. Significant radiation loss can occur from a fiber with such reduced core dimensions. In practice it is very difficult to control the stretching to obtain the desired coupling without suffering significant radiation loss in the coupling region.
Another device which employs optical fibers having a portion of the cladding removed to expose the core is a fiber optic polarizer. A fiber optic polarizer where a thin metallic film is formed on an exposed core portion of the optical fiber is disclosed in "In-Line Fiber Optic Polarizer", W. Eickhoff, Electronic Letters 16, page 762-763 (1980).
The fiber in the polarizer described by Eickhoff was prepared by grinding. This left a fiber with a curved base surface which hinders in line processing of long lengths of the fiber. If longer lengths could be readily processed, the region effecting polarization could be increased thus yielding larger degrees of polarization.